A. Field of the Invention
Aspects of the present invention relate generally to footwear. More particularly, the present invention relates to the use of a flexible plate as a support apparatus in an item of footwear.
B. Description of the Related Art
A major consideration in designing active footwear is hazards (e.g., stones and rocks). Stepping on stones, rocks or other small irregularities (hereinafter “rocks”) can concentrate forces on the shoe sole in a small area, thereby increasing pressure or stress in the area. A stone or rock is said to “penetrate” the sole when concentrated stresses are transmitted to cause discomfort to the wearer.
The extent of stress placed on a piece of footwear, and the associated risk of penetration, may be dependent on the size of the rock (i.e., small, medium, large etc.). Small rocks typically cause only limited concentrated stresses in a localized region of the sole of a shoe. See e.g., FIG. 1. Usually, these concentrated stresses are easily dissipated by using cushioning material in the shoe sole. Larger rocks also typically present lesser risk of penetration because larger rocks offer larger contact areas. See e.g. FIG. 2. Larger contact areas allow for a greater distribution (i.e., less concentration) of internal stresses created by contact, thereby minimizing risk of penetration.
Rocks of intermediate size, however, typically present the greatest risk of penetration. See e.g., FIG. 3. Stresses generated by intermediate rocks are typically concentrated enough to cause pain to the user. Rocks of intermediate size are those ranging from 1-6 centimeters.
Rock penetration is primarily a forefoot issue. The heel is typically protected by both a thicker cushion in the heel of a shoe, and a thick, fat pad of flesh located under the heel of the wearer. This multilayered cushion typically provides more than adequate protection against rock penetration, and serves to dissipate forces or stresses caused by contact. However, in the forefoot, hard tissues (e.g., bones) are closer to the ground surface, and less protected by cushioning soft tissue. Moreover, current shoe designs often require the shoe sole to be thinner in the forefoot, and therefore are less resistant to rock penetration.
Referring to FIG. 4, an exemplary shoe construction is shown. Boot 110 includes an outsole 112, midsole 114, shank 116, and shell 118. Heel cushion 120 and forefoot cushion 122 may be disposed between an insole 124 and shell 118. An upper 126 also may be provided, and optionally may include lacing 128. Preferably, shank 116 is disposed in a recess 114a in midsole 114, while cushions 120, 122 are disposed in recesses in insole 124.
In an exemplary embodiment, outsole 112 may be formed of carbon rubber, while midsole 114 may be formed of molded ethyl vinyl acetate foam. Shank 116 may be formed of thermoplastic polyurethane, while upper 126 may be formed of leather, fabric textiles, foam and other suitable insulation. Various polymer components may be coupled to each other with an adhesive or other bonding agent, while upper 126 may be coupled to shell 118, for example, using stitching proximate to the lower edge of leather portion 130 of upper 126.
A stiff plate may be inserted in the shoe to resist rock penetration. A plate presents a physical barrier to the rock, reducing the extent to which it compresses and penetrates the midsole. It further acts as a buffering conduit by redistributing concentrated stresses, thereby reducing internal stress in the sole materials and peak pressures acting on the foot.
The effectiveness of a plate design may be measured through various performance parameters. These performance parameters include flexibility, torsional flexibility/resistance, uniformity, weight, and stability. Flexibility may be measured by, among other things, the stiffness of the plate during bending. Torsional flexibility/resistance may be determined by, among other things, the stiffness of the plate during bending along the longitudinal axis of the plate. Uniformity typically refers to the ability of the plate to distribute the stresses incurred evenly across its body. Stability typically refers to the difference in stiffness between the medial/lateral and the central portions of the shoe.
In many instances, it may be desirable to provide a supportive, cushioning, rock-resistant plate design wherein the aforementioned criteria are optimized.